Cooling systems for turbocharged diesel engines of locomotives are known which provide for cooling of the engine and of a turbocharger aftercooler. One such system is described in U.S. Ser. No. 08/437,391, filed May 12, 1995 by Teoman Uzkan, one of the inventors of this invention, and issued as U.S. Pat. No. 5,598,705 on Feb. 4, 1997. This system provides separate engine and aftercooler coolant loops, each with its own radiator, coolant conduit and pump; but the system also provides a linking conduit with a link valve. When the valve is closed, coolant flows in the engine and aftercooler coolant loops are maintained separate, with no coolant mixing; but the valve is capable of opening to permit a controlled mixing of coolant between the loops. With the valve closed, the temperature in the aftercooler coolant loop is ordinarily lower than that in the engine coolant loop for maximum fuel economy due to the cooling of turbocharged induction air, but a control is capable of opening the link valve to provide additional cooling capacity in the engine coolant loop by mixing in the lower temperature coolant from the aftercooler coolant loop. Thus, economy can be achieved in the design of the engine coolant loop, with expansion of cooling capacity by the opening of the link valve in extreme temperature conditions when maximum engine cooling is required; but fuel economy can be maximized during normal temperature conditions by closing the link valve when maximum engine cooling is not required.